To create a Staff Message in Infinite Campus, on the Index go to Messenger > Staff Messenger.
If using a template find the template, otherwise proceed to create the message. Once created, the message can be saved.
Change the Message Type: to Priority Notification in the drop down. For the Deliver To: set to All Staff of School if it is a school sending the message. If it is not, set to Ad Hoc Filter for specific recipients in an already created Ad Hoc Filter.
Find the Ad Hoc Filter of the recipients who should receive the message. If needed, more than one can be selected.

Select the Deliver Devices. Choose all devices as recipients. The user account Demographic Tab will define how they will receive the message.
Create the Message.

If necessary, Record a Message, or upload a recorded message or an email attachment.
Next, create the Text Message. Feel free to copy and paste from the email window.
Set the Delivery Date and Time for the delivery of the message. Additionally set the Dial Window if there is a voice message. The until time can be an hour after the Dial Window. Note: It will happen much quicker than that.
Click on Preview/Send.
The Preview Message window will come up showing the number of recipients. If correct, click on Send Message.